System and method for measuring velocity using frequency modulation of laser output

ABSTRACT

The invention relates to a system for measuring velocity. In one aspect of the invention, the system comprises a laser emitting device, an optics assembly, a combiner and a signal processor. The laser emitting device generates a frequency-modulated laser signal. The optics assembly splits the frequency modulated signal into a plurality of laser signals, and directs the split signals to a target and receives at least one signal reflected from the target. The combiner receives one of the split signals transmitted via one signal path and the at least one reflected signal transmitted via another signal path, and multiplies the received signals. The signal processor extracts velocity information from the multiplied signal.

RELATED APPLICATIONS

[0001] This application claims priority under 35 U.S.C. § 119(e) fromU.S. provisional application No. 60/384,526 filed May 29, 2002, which ishereby incorporated by reference.

BACKGROUND

[0002] 1. Technical Field

[0003] The invention relates to a velocity measurement system, andparticularly to a system and method for measuring the direction andmagnitude of the velocity of a target by performing frequency modulationof a laser output

[0004] 2Description of the Related Technology

[0005] A number of systems and apparatuses have been developed formeasuring the distance and velocity of an object using various methods.

[0006] As one example of the systems, U.S. Pat. No. 6,133,993 discloses“a length and velocity measurement apparatus.” The apparatus disclosesusing amplitude modulation (AM) and Doppler shift of carrier in order tomeasure velocity.

[0007] As another example of the systems, U.S. Pat. No. 6,317,073discloses “FW-CW radar system for measuring distance to and relativespeed of a target.” The system measures the speed and distance of thevehicle using a radar wave.

[0008] In addition to the above patents, many other U.S. patents, suchas 6,311,121, 6,141,086, 5,164,784 and 3,915,572, etc., discuss methodsfor measuring speed of a target.

SUMMARY OF CERTAIN INVENTIVE ASPECTS OF THE INVENTION

[0009] One aspect of the invention provides a system for measuringvelocity. The system comprises a power source, a laser emitting device,a frequency modulating section, an optics assembly, a combining section,a detector and a signal processor. The power source provides a current,and the laser emitting device is powered by the current and configuredto emit a laser signal. The frequency modulating section frequencymodulates the laser signal. The optics assembly splits the frequencymodulated signal into a plurality of laser signals, and directs thesplit signals to a target and receives at least one signal reflectedfrom the target. The combining section combines one of the split signalstransmitted via one signal path and the at least one reflected signaltransmitted via another signal path. The detector detects the combinedsignal. The signal processor extracts velocity information from thedetected signal.

[0010] In this aspect of the invention, the laser emitting device andthe detector are configured together in a laser assembly. The frequencymodulating section is located in the laser assembly The combiningsection receives the one of the split signals and the at least onereflected signal, and multiplies the received signals so as to providethe combined signal.

[0011] The laser emitting device comprises a laser diode. The signalprocessor extracts distance information toward the target from thevelocity information. The system further comprises a quadrature mixerconfigured to process the detected signal adapted to be used in thesignal processor, wherein the signal processor is a conventional Dopplersignal processor. The extracted velocity information comprises themagnitude and direction of the velocity of the target.

[0012] Another aspect of the invention provides a method of measuringvelocity. The method comprises emitting a laser signal, frequencymodulating the laser signal, and splitting the frequency modulatedsignal into a plurality of laser signals. The method also comprisesdirecting the split signals to a target, receiving at least one signalreflected from the target, and combining one of the split signalstransmitted via one signal path and the at least one reflected signaltransmitted via another signal path. The method further comprisesdetecting the combined signal and extracting velocity information fromthe detected signal.

[0013] In this aspect of the invention, the combining comprises samplingone of the split signals and at least one reflected signal, andmultiplying the sampled signals. The multiplying comprises summing thesampled signals and squaring the sum. The method further comprisesgenerating a voltage signal that is proportional to the multipliedsignal. The extracting comprises processing the voltage signal so as toobtain a Doppler frequency including sign. The method further comprisescalculating distance information toward the target from the velocityinformation.

[0014] Another aspect of the invention provides a system for measuringvelocity. The system comprises a laser emitting device, a frequencymodulating section, an optics assembly, a detector and a signalprocessor. The laser emitting device emits a laser signal. The frequencymodulating section performs frequency modulation on the emitted lasersignal. The optics assembly splits the frequency modulated signal into aplurality of laser signals, and directs the split signals to a targetand receives at least one signal reflected from the target. The detectordetects a combined signal of one of the split signals transmitted viaone signal path and the at least one reflected signal transmitted viaanother signal path. The signal processor extracts velocity informationfrom the detected signal.

[0015] In this aspect of the invention, the laser emitting device andthe detector are configured together in a laser assembly. The detectorcomprises a detector diode. The system further comprises a sinusoidalwave generator configured to generate a sinusoidal wave, wherein thefrequency modulating section is configured to frequency modulate thelaser signal with the sinusoidal wave.

[0016] Another aspect of the invention provides a system for measuringvelocity. The system comprises a laser emitting device, an opticsassembly, a combiner and a signal processor. The laser emitting devicegenerates a frequency-modulated laser signal. The optics assembly splitsthe frequency modulated signal into a plurality of laser signals, anddirects the split signals to a target and receives at least one signalreflected from the target. The combiner receives one of the splitsignals transmitted via one signal path and the at least one reflectedsignal transmitted via another signal path, and multiplies the receivedsignals. The signal processor extracts velocity information from themultiplied signal.

[0017] In this aspect of the invention, the laser emitting device andthe combiner are configured together in a laser assembly. The combinercomprises a detector.

[0018] Another aspect of the invention provides a method of measuringvelocity. The method comprises generating a frequency-modulated lasersignal, splitting the frequency modulated signal into a plurality oflaser signals, directing the split signals to a target and receiving atleast one signal reflected from the target. The method also comprisescombining one of the split signals transmitted via one signal path andthe at least one reflected signal transmitted via another signal path,and extracting velocity information from the combined signal.

[0019] In this aspect of the invention, the combining comprisesreceiving the one of the split signals and the at least one reflectedsignal, and multiplying the received signals. The method furthercomprises detecting the combined signal, wherein the extracting is fromthe detected signal. The method further comprises extracting distanceinformation toward the target from the velocity information.

[0020] Another aspect of the invention provides a system for measuringvelocity. The system comprises a laser device, a detector and a signalprocessor. The laser device generates a frequency-modulated lasersignal, wherein the frequency modulated signal is directed to a targetand reflected from the target. The detector detects a combined signal ofthe frequency-modulated laser signal transmitted via one signal path andat least one reflected signal transmitted via another signal path. Thesignal processor extracts velocity information from the detected signal.

[0021] In this aspect of the invention, the laser device and thedetector are configured together in a laser assembly. The system furthercomprises a combiner configured to receive the frequency-modulated lasersignal and the at least one reflected signal and to multiply thereceived signals so as to provide the combined signal. The detectorproduces a voltage signal that is proportional to the multiplied signal.The detector comprises a detector diode. The system further comprises anoptics assembly configured to split the frequency modulated signal intoa plurality of laser signals, and directs the split signals to thetarget and receives at least one signal reflected from the target. Theoptics assembly comprises a power splitter configured to split thefrequency-modulated laser signal into first and second split signalshaving different signal paths from each other, and a collimatorconfigured to collimate and direct the first split signal to the target,and to collimate and direct a reflected laser signal to the detector,wherein the detector is configured to detect a combined signal of thereflected laser signal and the second split signal. The system furthercomprises a circulator configured to receive and route the first splitlaser signal toward the collimator and to receive and route thereflected signal toward the detector. The collimator comprises a firstcollimator configured to collimate and direct the first split signal tothe target, and a second collimator configured to collimate and direct areflected laser signal to the detector. The system further comprises aswitch configured to switch the first split laser signal between thepower splitter and the first and second collimators.

[0022] Another aspect of the invention provides a method of measuringvelocity. The method comprises generating a frequency-modulated lasersignal, wherein the frequency modulated signal is directed to a targetand reflected from the target. The method also comprises combining thefrequency-modulated laser signal transmitted via one signal path and atleast one reflected signal transmitted via another signal path, andextracting velocity information from the combined signal.

[0023] In this aspect of the invention, the combining comprises samplingthe frequency-modulated laser signal and at least one reflected signal,and multiplying the sampled signals so as to provide the combinedsignal. The multiplying comprises summing the sampled signals andsquaring the sum. The method further comprises producing a voltagesignal that is proportional to the multiplied signal, wherein theextracting is from the produced voltage signal. The method furthercomprises detecting the combined signal.

[0024] Another aspect of the invention provides a method of measuringvelocity. The method comprises frequency modulating a laser signal, thefrequency modulated signal being split into a plurality of laser signalsand directed to a target, and reflected from the target. The method alsocomprises receiving one of the split signals transmitted via one signalpath and at least one reflected signal transmitted via another signalpath, multiplying the received signals and extracting velocityinformation from the multiplied signal.

[0025] In this aspect of the invention, the multiplying comprisessumming the received signals and squaring the sum. The method furthercomprises producing a voltage signal that is proportional to themultiplied signal, wherein the extracting is from the produced voltagesignal. The method further comprises filtering an amplitude modulationcomponent that has been incidentally generated.

[0026] Another aspect of the invention provides a system for measuringvelocity. The system comprises a laser-emitting device, a frequencymodulating section, a power splitter, a combiner and a signal processor.The laser-emitting device emits a laser signal. The frequency modulatingsection performs frequency modulation on the laser signal, wherein thefrequency-modulated laser signal is directed toward a moving target. Thepower splitter splits the frequency-modulated laser signal into firstand second laser signals, wherein one of the laser signals is delayedwith respect to the other. The combiner receives and combines reflectedsignals from the target so as to provide a combined signal. The signalprocessor extracts velocity information from the combined signal. Inthis aspect of the invention, the system further comprises a detectorconfigured to detect the combined signal.

[0027] Another aspect of the invention provides a method of measuringvelocity. The method comprises frequency modulating a laser signal, thefrequency modulated signal being split into a plurality of laser signalsand directing the split laser signals to a target. The method alsocomprises combining one of the split laser signals with a reflectedsignal from the target, and extracting velocity information from thecombined signal. The combining comprises receiving the one of the splitlaser signals and the reflected signal, and multiplying the receivedsignals so as to provide the combined signal. In this aspect of theinvention, the method further comprises detecting the combined signal.

[0028] Still another aspect of the invention provides a system formeasuring velocity. The system comprises a laser assembly and a signalprocessor. The laser assembly generates a frequency-modulated lasersignal, the frequency-modulated signal being split into a plurality oflaser signals and directed to a target, and reflected from the target,wherein the laser assembly receives one of the split signals transmittedvia one signal path and at least one reflected signal transmitted viaanother path, and combines the received signals. The signal processorextracts velocity information from the combined signal.

[0029] In this aspect of the invention, the laser assembly comprises alaser emitting device configured to emit a laser signal, a frequencymodulating section configured to perform frequency modulation on theemitted laser signal, and a combiner configured to receive and combinethe one of the split signals and the at least one reflected signal.Alternatively, the laser assembly comprises a laser emitting deviceconfigured to emit a laser signal, a frequency modulating sectionconfigured to perform frequency modulation on the emitted laser signaland a detector configured to receive the one of the split signals andthe at least one reflected signal, and to multiply the received signalsso as to provide the combined signal.

[0030] Still another aspect of the invention provides a system formeasuring velocity. The system comprises means for generating afrequency-modulated laser signal, the frequency modulated signal beingsplit into a plurality of laser signals and directed to a target, andreflected from the target and means for receiving one of the splitsignals transmitted via one signal path and at least one reflectedsignal transmitted via another signal path, and means for multiplyingthe received signals, and means for extracting velocity information fromthe multiplied signal.

[0031] Yet another aspect of the invention provides a method ofmeasuring velocity. The method comprises generating afrequency-modulated laser signal, the frequency-modulated signal beingdirected to a target, and measuring the velocity of the target based ona combination of the frequency-modulated laser signal and a reflectedsignal from the target.

[0032] In this aspect of the invention, the measuring the velocitycomprises measuring the velocity of one of the following targets: amoving vehicle, a cable being extruded from a die, a sheet metal beingrolled through a roller, a cable being transferred between two spools, asurface of liquid, a vibrating object, a rotating machinery, or a groundmoving with respect to a vehicle. The method further comprises obtainingthe distance toward the target from the measured velocity. The measuringthe velocity comprises measuring a closing velocity of two aircrafts ormeasuring a ground velocity of an aircraft or a missile. The measuringcomprises sensing a ground velocity of the target on the order ofmicrometers per second at acoustic frequencies from 50 Hz to 1 kHz. Themethod further comprises using interferometic or heterodyne sensing ofthe reflected laser signal.

[0033] Yet another aspect of the invention provides a method ofmeasuring velocity and distance. The method comprises generating afrequency-modulated laser signal, the frequency-modulated signal beingdirected to a target, measuring the velocity of the target based on acombination of the frequency-modulated laser signal and a reflectedsignal from the target and obtaining the distance to the target by themeasured velocity. In this aspect of the invention, the measuringcomprises measuring the speed and direction of motion of a groundvehicle that aids a fire control or navigation system, or permits deadreckoning.

BRIEF DESCRIPTION OF THE DRAWINGS

[0034]FIG. 1 illustrates a block diagram of a Doppler radar system.

[0035]FIG. 2 illustrates a block diagram of the velocity measurementsystem according to one embodiment of the invention.

[0036]FIG. 3 illustrates a block diagram of the velocity measurementsystem according to another embodiment of the invention.

[0037]FIG. 4 illustrates a block diagram of the velocity measurementsystem according to still another embodiment of the invention.

[0038]FIG. 5 illustrates a block diagram of the velocity measurementsystem according to yet another embodiment of the invention.

[0039]FIG. 6 illustrates a configuration for measuring the rate at whicha cable is extruded through a die in one application of the invention.

[0040]FIG. 7 illustrates the use of a single beam to measure the amountor rate of material being such as metal using rollers in anotherapplication of the invention.

[0041]FIG. 8 illustrates the use of a beam to measure the speed and/oramount of a cable being transferred between two spools in anotherapplication of the invention.

[0042]FIG. 9 illustrates a beam used to measure the flow of a liquid ina channel in another application of the invention.

[0043]FIG. 10 illustrates a laser Doppler system mounted to a vehicle inanother application of the invention.

[0044]FIG. 11 illustrates an arrangement of three lasers and theiroptics pointing in three different directions to get the threecomponents of velocity in another application of the invention.

[0045]FIG. 12 illustrates an alternate arrangement of three lasers andtheir optics pointing in three different directions to get threecomponents of velocity in another application of the invention.

[0046]FIG. 13 illustrates an arrangement of two lasers and their opticspointing in two different directions to get two components of velocityin another application of the invention.

[0047]FIG. 14 illustrates a four beam system which is over determinedand permits an error velocity to be estimated in another application ofthe invention.

DESCRIPTION OF CERTAIN INVENTIVE EMBODIMENTS

[0048] There are many Doppler velocimeters that measure the velocity ofa target using the Doppler effect. Among them, some Doppler velocimetersuse a laser, which will be hereinafter referred to as “laser Dopplervelocimeters” for convenience. Laser Doppler velocimeters can beclassified as either a Type 1 system or a Type 2 system.

[0049] The Type 1 system measures the speed of the target moving toward(or away) from the system along the beam. The laser output is split intotwo beams. One beam is aimed at the target and is reflected by it. Theenergy reflected by the target is collected and added to the otherreference beam. The total beam is applied to the detector(s). No motioncreates a zero frequency output at the detector. Equal motion in eitherdirection causes the same output of the detector. The Doppler frequencyis then determined by processing the detector output by some type ofsignal processor. The Type 1 system comprises Classes A-G. Some of theClasses relate to the approach for determining the direction of themotion.

[0050] The Type 1 Class A system does not attempt to determine thedirection of motion.

[0051] The Type 1 Class B system is built with a Bragg Cell in serieswith one of the two beams, usually the reference beam. The Bragg Cell isused to offset the frequency in the beam by a precisely known amount.The result is that the “zero Doppler” frequency at the output of thedetector is the frequency offset created by the Bragg Cell. A targetvelocity toward system further offsets this frequency one way and motionin the other direction offsets the frequency in the other direction.This system can tell the direction of motion.

[0052] In the Type 1 Class C system, the frequency of the laser is sweptup in frequency (chirped) and then swept down. The sweeps are treated asif they were two independent steps. Since the path length directly fromthe laser to the detector is different (shorter) from the path lengthfrom the laser to the detector via the target, the frequency during eachsweep at the detector output will be proportional to the product of thesweep rate and path length difference with the Doppler frequency added.By combining the measurements of the frequency at the output of thedetector during each of the sweeps, the Doppler frequency, includingdirection information, may be determined. For instance, if the sweeprates are equal but opposite in sign, the Doppler frequency is half ofthe sum of the frequency output during each sweep. The difference in thetwo path lengths can be found by adding the two measurements as well.This system can therefore determine both the range to the target and thespeed of the target to or from the system. The problem with this systemis that the sweep rate must be precisely controlled. If it is not, anerror of the Doppler shift is created. This is very hard to do with alaser.

[0053] The Type 1 Class D system uses 2 “edge filters” to separate thepositive and negative Doppler frequencies. The reflected beam from thetarget and the reference beam are split into two beams, respectively,one of the beams from the target going through a filter with the loweredge of its passband at the zero Doppler frequency. Positive Dopplerfrequencies pass through this filter and negative ones are rejected.After passing through the filter the beam is added to one of thereference beams and applied to the detector. This detector is used forthe positive Doppler. The other beam from the target passes through afilter whose upper edge of its passband is at the zero Dopplerfrequency. This filter rejects positive Doppler frequencies and passesnegative Doppler frequencies. After passing through the filter it isadded to the reference beam and applied to the detector. This detectoris used for the negative Doppler. Thus, the direction of motion isdetermined. This system has a problem in that it is difficult toprecisely set the edge of the filters at the zero Doppler frequency andensure that there are no drift issues. Also any gain slope in thepassband of the filters may induce an error in the Doppler frequencymeasurement.

[0054] The Type 1 Class E system is similar to the Type 1 Class Asystem, but in this system there are actually two detectors. Before thereference beam and the beam from the target are added, each of them issplit into two beams. Each one is added and applied to a detector. Thepath length involved in the two paths to the detector differs in the twocases by 90 degrees. The two detector outputs form a quadrature pairthat permits the determination of the direction of motion as well at themagnitude. This approach is common in radar and sonar, but for lasersystems it is hard to create and maintain the path length differencesbecause they are so small.

[0055] The Type 1 Class F system passes the reflected energy through adevice that permits the frequency of the reflected energy to be measuredoptically. The received energy is passed through some type of filterthat converts frequency to amplitude, which is measured as an analog forfrequency.

[0056] The Type 1 Class C system uses the laser as both the light sourceand detector/mixer. The reflected light from the target re-enters thelaser. The laser mixes the light in the laser with the reflected lightto produce the difference frequency. This is commonly performed withsemiconductor lasers because they actually have a detector diode tomeasure the laser output power that can be used as a mixer/detector (see“Laser Doppler Velocimeter employing the laser as mixer-oscillator,”Rudd, J. Phys. E 1, 1968, 723-726 or “Laser Doppler Velocimeter usingthe self-mixing effect of semiconductor laser diode,” Shinohara, et al.,Applied Optics 25, May 1, 1986, 1417-1419) but can also be done withother laser types as well. If the light reflected from the target intothe laser is strong enough, it will change the operation of the laserand distort the shape of the waveform generated from the mixer. The newwaveform shape is a sawtooth and the direction of the motion can bedetermined from the sawtooth shape. This scheme for direction sensingrequires that the reflected signal be strong, not found in most systemapplications.

[0057] The Type 2 system measures velocity across the beam (not alongit) and has the characteristics that the laser output is split into twobeams. Both beams are aimed so as to be nearly parallel and intersect atthe target in the same spot. This creates an interference pattern on thetarget. The reflected energy from the target is collected and applied toa detector. The Doppler frequency is then determined by processing thedetector output by some type of signal processor. The Type 2 systemcomprises Classes A-C.

[0058] In the Type 2 Class A system, the laser output is split into two(equal power) beams. Both beams hit the target in the same spot, butfrom slightly different directions, which creates an interferencepattern on the target. Some of the reflected energy from the target iscollected and applied to a detector. When there is motion of the targetacross the interference pattern, the energy hitting the detectorfluctuates. From the frequency of the detector output the motion of thetarget can be determined. This system is unable to tell the direction ofthe motion.

[0059] In the Type 2 Class B system, a Bragg Cell in series with one ofthe two arms of the Type 2 Class A system is added. The result is thatthe interference pattern on the target sweeps across the target, evenwhen the target is stationary. This creates an offset in the output ofthe detector, similar to that in the Type 1 Class B system. The resultis that the “zero Doppler” frequency at the output of the detector isthe frequency offset created by the Bragg Cell. Target velocity acrossthe interference pattern further offsets this frequency one way andmotion in the other direction offsets the frequency in the otherdirection. Thus this system can tell the direction of motion.

[0060] The Type 2 Class C system is similar to the Type 2 Class Asystem. There is a delay added to one of the two arms and the laserfrequency is swept up and then down. The sweeps are treated as twoindependent steps. Because of the delay and the frequency sweep, thefrequencies out of the two arms landing on the target are different andthe interference pattern on the target slides one way during one sweepand the other during the other sweep. The direction of motion can bedetermined using a method similar to that of the Type 1 Class C system.The problem with this system is that the sweep rate must be preciselycontrolled. This is very hard to do with a laser. If it is not, an errorof the Doppler shift is generated.

[0061] However, the above systems have a complicated velocimeter or needcomponents that are expensive or requiring significant power.

[0062] There is also a traditional Doppler radar system that measuresthe velocity of a target. This system has separate antennas for transmitand receive sections. The received signal is mixed, using quadraturemixers, with a sample of the transmit signal. The result of the mixingoperation is to give a pair of signals (one called “real” the other“imaginary”) that, in combination can be used to determine both theDoppler shift (velocity) and direction. If the real and imaginarysignals are plotted against one another and there is a Doppler shift,over time, they will trace out a circle. The direction traced out(clockwise vs. counter- clockwise) gives the Doppler shift (velocity)direction and the number of circles drawn per second gives the Dopplershift (velocity) magnitude. This parallels Type 1 Class E system.

[0063] There is also a type of Doppler radar system that is built verydifferently from the traditional system. Most laser systems and thisDoppler radar system have only a single mixer, which means that theabove technique used in the traditional radar system does not work.

[0064]FIG. 1 illustrates a simplified block diagram of the Doppler radarsystem. There is an oscillator 110, operating at, for example, 13.3 GHz,which is used to generate the transmit power and local oscillatorfrequency (reference frequency for a quadrature mixer 150). Theoscillator 110 has a frequency control that is used tofrequency-modulate the oscillator frequency between two frequencies, forexample, 450 KHz apart. The goal of the frequency shifting is to put thezero Doppler frequency at the output of an image rejection mixer 140 toa nonzero value (in this case 450 KHz) so that the sign of the Dopplerfrequency can be easily determined. To use one antenna 160 for bothtransmitting and receiving, it is time shared between the two functions.The transmit and receive switches 120 and 130 are used to control whichof these functions the antenna 160 is being used for. The frequency ofthe oscillator 110 is switched simultaneously with the transmit/receiveswitches 120 and 130. One is frequency for transmitting and the otherfor receiving. The idea is that the same oscillator 110 is used for boththe transmit power and the local oscillator power, at a differentfrequency from transmitting, during receiving. This puts the zeroDoppler frequency out of the image rejection mixer 140 at the differencebetween the two oscillator frequencies.

[0065] The output of the image rejection mixer 140 is amplified andbandpass filtered (not shown) and applied to the quadrature mixer 150.The output of this mixer 150 can be used as in a standard system todetermine the Doppler frequency and its sign.

[0066] However, the difference between the two different frequenciesgenerated from the oscillator 110 cannot be precisely controlled. Thiswould mean that one would not know precisely the zero Doppler frequencyat the output of the image rejection mixer 140, creating an unacceptableerror. A few Hertz error would be significant. Seemingly, this systemcould not possibly work, but it does.

[0067] After mathematical analysis of these systems, it becomes apparentthat the zero Doppler frequency is determined by the frequency that isused to control the transmit/receive and frequency control. Onlyfrequencies that are harmonics of that switching rate can exist. Theoscillator frequency shift only controls the power distribution betweenthe harmonics of the switch rate. If the laser frequency shift driftsfrom the desired operating frequency shift, it does not create ameasurement error, it just changes the signal power and the maximumdistance the target may be from the system.

[0068] One embodiment of the invention is a velocity measurement systemthat can sense both the direction and magnitude of motion by combiningthe Doppler radar system with a laser based velocity measurement system.Specifically it relates to how to sense that direction of motion withoutthe addition of Bragg cells or other similar devices.

[0069] One embodiment of the invention is used to measure the relativemotion between the device and a target. With appropriateimplementations, separation between device and target can be a fractionof a meter to 100's or 1000's of meters. Even though systems having onlyone beam are illustrated, most applications may have at least two andusually three or four and possibly five or six beams. It is understoodthat the additional beams are implemented as the first beam, probablysharing some or most of the implementation.

[0070] In one embodiment, as illustrated in FIG. 2, a laser diode 230 isused as both the source of the energy and the detector as described in“Laser Doppler Velocimeter using the self-mixing effect of semiconductorlaser diode,” Shinohara, et al., Applied Optics 25, May 1, 1986,1417-1419. Many laser diode assemblies contain both a laser diode and adetector diode that is intended to monitor the output of the laserdiode. That diode may also be used as a detector of laser energy of thereceived signal. A constant current power supply 210 provides a currentto power the laser diode 230. The voltage across the diode 230 ischanged by the received optical energy. Usually this is to be avoidedand isolators are used to prevent this from happening by blocking thereflected energy. Here some advantages of this effect are being taken.

[0071] In one embodiment, the laser diode 230 includes a VCSEL (VerticalCavity Surface Emitting Laser) available from Honeywell. This diodeoperates at 870 nm and the wavelength gives a sensitivity scale factorof 435 nanometers/second/Hertz of Doppler shift.

[0072] The laser diode 230 is powered by a DC current source providedfrom the constant current power supply 210. In one embodiment, the powerof the laser diode 230 is limits the maximum separation between thisvelocimeter system and the target to several inches, possibly a foot.

[0073] The provided current is modulated by a small (parts per millionof the DC current source) amplitude sine wave generated in a sine wavegenerator 220. This sine wave is used by the diode 230 to amplitudemodulate (AM) and frequency modulate (FM) the laser light signal. Thesine wave also changes the voltage at the electrical input to the diode230. In this case, the amplitude modulation and the voltage change arenuisances and the frequency modulation is desired. However, in mostother communication applications, the amplitude modulation would bedesired and the modulation much greater. Thus, one embodiment of theinvention ignores the amplitude modulation and the voltage change of thediode 230. Changing the current in the diode 230 changes the chargedensity in the diode 230, which changes the speed of light and thelasing frequency.

[0074] In other inventive embodiments, a separate frequency-modulatingsection may be provided and located either inside or outside of thelaser diode 230.

[0075] The laser optical output is focused on a target 250 by an opticsassembly 240, which reflects the light. Some of the reflected energy iscollected by the optics assembly 240 and focused back on the laser diode230. This light enters the laser diode 230 and influences its operation,which changes the voltage at the electrical input of the laser diode230. If a monitor diode is available and used for detection, both thelight from the laser diode 230 and the target 250 must land in the samespot on the detector and from the same direction. The detected “output”is then processed in a typical Doppler signal processor 270 to determinethe velocity of the target 250. The processed signal is output ordisplayed through an output/display 280. Reference numeral 260represents the direction of the velocity of the target 250.

[0076] For the small amount of sinusoidal current used to modulate thediode 230, it can be assumed that the voltage created by the current atthe diode electrical input and the amplitude modulation are alsosinusoidal. It is assumed that the frequency of the sine wave is F.Then, since modulation signal is very small, the amplitude modulationand the voltage change are also a signal with frequency F. This meansthat there are no harmonics thereof, 2F, 3F, 4F, etc. generated in theamplitude modulation and the voltage change. This is not true of FM,which generates many harmonics. With sinusoidal modulation the FMharmonics are determined by Bessel functions.

[0077] Mathematically the following is happening. The output of thefrequency modulated laser diode 230 can be represented as.

X(t)=Cos(ω_(c) t+φ(t))  Equation 1

[0078] The transmitted signal is delayed by t_(D) as it travels to thetarget 250 and back and mixed with the (non-delayed) transmitted signal.The result is

R(t)=X(t)X(t−t _(D))=Cos(ω_(c) t+φ(t))Cos((ω_(c)+ω_(D)))(t−t _(D))+φ(t−t_(D)))  Equation 2

[0079] where R(t) is the result of the mixing operation, ω_(c) is thecarrier frequency, ω_(D) is the Doppler shift and φ(t) is the phasemodulation.

R(t)=COS(ω_(D) t+φ(t−t _(D))−φ(t))  Equation 3

[0080] after the high frequency terms are discarded and ignoringω_(D)t_(D), which is a random phase shift.

[0081] Assume that ω(t)=βCos(ω_(m)t), then $\begin{matrix}{{R(t)} = {{{{Cos}\left( {\omega_{D}t} \right)}{{Cos}\left( {\beta_{1}{{Sin}\left( {\omega_{m}\left( {t - {t_{D}/2}} \right)} \right)}} \right)}} - {{{Sin}\left( {\omega_{D}t} \right)}{{Sin}\left( {\beta_{1}{{Sin}\left( {\omega_{m}\left( {t - {t_{D}/2}} \right)} \right)}} \right)}}}} & {{Equation}\quad 4}\end{matrix}$

[0082] and (see any book on modulation theory or Reference Data forRadio Engineers: Radio, Electronics, Computer and Communications,Indianapolis, IN, Howard W. Sams & Co., 1985, p. 46-39) $\begin{matrix}{{R(t)} = {{{{Cos}\left( {\omega_{D}t} \right)}\left( {{J_{0}\left( \beta_{1} \right)} + {2{\sum\limits_{n = 1}^{\infty}\quad {{J_{2n}\left( \beta_{1} \right)}{{Cos}\left( {2n\quad {\omega_{m}\left( {t - {t_{D}/2}} \right)}} \right)}}}}} \right)} - {2{{Sin}\left( {\omega_{D}t} \right)}\left( {\sum\limits_{n = 1}^{\infty}\quad {{J_{{2n} - 1}\left( \beta_{1} \right)}{{Sin}\left( {\left( {{2n} - 1} \right){\omega_{m}\left( {t - {t_{D}/2}} \right)}} \right)}}} \right)}}} & {{Equation}\quad 5}\end{matrix}$

[0083] Notice that the COS(ω_(D)t) terms are associated with even orderharmonics and Bessel orders and the Sin(ω_(D)t) terms with odd harmonicsand Bessel orders. It is assumed for the convenience that the zero order(harmonic) is ignored because it can get confused with the DC bias onthe diode. It is also assumed that the first order (harmonic) is ignoredbecause it will be confused with the modulating signal.

[0084] Using the second and third orders gives:

R(t)=2J ₂(β₁)Cos(ω_(D) t)Cos(2ω_(m)(t−t _(D)/2)) −2J ₃(β₁)Sin(ω_(D)t)Sin(3ω_(m)(t−t _(D)/2))  Equation 6

[0085] Now provide two local oscillators Cos(2ω_(m)), (t−t_(D)/2)) andSin(3ω_(m)(t−t_(D)/2)) that are separately mixed with R(t) (and the highfrequency terms ignored)

Re(t)=Cos(2ω_(m)(t−t _(D)))R(t)=2J ₂(β₁)Cos(ω_(D)t)Cos(2ω_(m)(t−t _(D)/2))² =J ₂(β₁)Cos(ω_(D) t)  Equation 7

Im(t)=Sin(3 ω_(m)(t−t _(D)))R(t)=2J ₃(β₁)Sin(ω_(D) t)Sin(t−t _(D)/2))²=J ₃(β₁)Sin((ω_(D) t)  Equation 8

[0086] Equations 7 and 8, respectively, represent a complex pair neededto do the Doppler processing. The Doppler processing may be done anynumber of ways from here. Usually Re(t) and Im(t) are combined into acomplex channel Re(t)+jIm(t) (j=−1) and processed.

[0087] One embodiment of the invention calculates the complexautocorrelation,

(τ_(1.)), function at some convenient lag, τ_(L), and calculates theDoppler frequency from ƒ_(D)=tan⁻¹(Im(

(τ_(1.)))/Re( (τ_(1.))))/2ππ_(1.). See Miller, et al., “A CovariancSpectral Moment Estimation”, IEEE Transactions on Information Theory,Sep. 1972, pp. 588-596.

[0088] In one embodiment, the Doppler signal processor 270 comprises aconventional Doppler signal processor. In this embodiment, the systemcomprises a quadrature mixer (as shown in FIG. 1) between the laserdiode 230 and the Doppler signal processor 270. The quadrature mixerprocesses an emitted (frequency-modulated) signal and a reflected(detected) signal received from the laser diode 230, and provides asignal being suitable for use in the conventional Doppler signalprocessor as discussed below.

[0089] If the frequency modulating sine wave is Cos(ω_(m)t) then, toobtain the real part, the quadrature mixer multiplies the detectedsignal by Cos[2ω_(m)(t−t_(d))] (see Equation 7) and to obtain theimaginary part it multiplies the detected signal by Sin[3ω_(m)(t−t_(d))](see Equation 8). t_(d) is the round trip delay time between thefrequency-modulating section of the laser diode 230 and the target 250.In one embodiment, the time t_(d) is small enough and can be ignored.

[0090] In one embodiment, t_(d) could be used to determine the distancetoward the target 250. By adjusting t_(d) of Sin[2ω_(m)(t−t_(d))] andCos[3ω_(m)(t−t_(d))] (note that the sine and cosine have beeninterchanged) until the resulting signals are nulled, an estimate of therange can be obtained. In one example, the range is t_(d)xc/2 where c isthe speed of light.

[0091] Alternatively, as shown in FIG. 2, the Doppler signal processor270 may not need a quadrature mixer and may directly obtain velocityinformation from an emitted (frequency-modulated) signal and a detectedsignal received from the laser diode 230.

[0092] The invention may be embodied to various systems, discussedabove, which measure the magnitude and direction of velocity as follows.

[0093] Example 1 represents velocity interpretation using the Type 1system in a very short range. In Example 1, it is supposed that a laserwith a wavelength of 635nm is used so that it can easily be seen if thebeam is on the target.

[0094] In Example 1, since the maximum velocity is 50 m/s, the maximumDoppler frequency is 3.15 MHz/(m/s)×50(m/s)=157 MHz. In order to avoidthe confusion of one of the FM harmonics with the Doppler, the minimummodulating frequency, F in above, is defined twice that, 315 MHz. Betais the ratio of the frequency deviation of the FM to the modulatingfrequency. The second and third harmonics of a sinusoidal FM process areequal at Beta of approximately 3.77 (Betal). There is a relation betweenBeta and Betal, β₁=2βSin(ω_(m)t_(D)/2)=2βSin(ω_(m)d_(D)/c), where d_(D)is the target distance. Knowing that Betal is 3.77 and the other factorsin the equation, Beta=3.07. Thus if the minimum modulating frequency is315 MHz, then the deviation must be 3.07×315 MHz=968.5 MHz. Thiscorresponds to 2 ppm (parts per million) of the laser's frequency.EXAMPLE 1 Doppler Data For 635 nm Laser Laser-Target Range, m 0.1Wavelength, nm= 635 Maximum Velocity, m/s 50 Result Doppler Scalefactor, MHz/m/s 3.15 Minimum Modulating Frequency, MHz 315.0 Beta 3.07Deviation, MHz 968.5 Deviation, ppm 2.0 Beta1 3.77

[0095] The Doppler scale factor (SF) is determined from the laserwavelength, SF=2/λ. Assuming the velocity measurement range is twice themaximum velocity, the ranges of Doppler frequency range are2(SF)V_(max)=4V_(max)/λ. The modulation frequency, F_(m), ω_(m)=2πF_(m),is at least this amount.

[0096] Example 1 provides an opportunity to point out that a “tunable”laser is not required. The word tunable means that a laser frequency maybe tuned over a significant frequency (wavelength) range. Only two partsin a million is required here. This is easily accomplished by smallmodulation of the operating current of a laser diode or the currentdriving other lasers that are powered by current flow, such as HeliumNeon. For instance, Honeywell characterizes its VCSEL laser as having awavelength tuning sensitivity of dl/dl˜0.09 nm/mA, which is equivalentto 100 ppm/mA, even though Honeywell would not call their product“tunable”. The laser frequency changed by changing the drive current istypically a problem in communication systems, because as the drive ischanged to turn the laser on an off, the frequency is also sweptcreating a “chirp” effect.

[0097] Example 2 represents velocity interpretation using the Type 1system in a long range. In Example 2, a CO₂ laser is used. EXAMPLE 2Doppler Data For 10600 nm (CO₂) Laser Laser-Target Range, m 1000wavelength, nm= 10600 Maximum Velocity, m/s 50 Result Doppler Scalefactor, MHz/m/s 0.19 Minimum Modulating Frequency, MHz 18.9 Beta 3.03Deviation, MHz 57.1 Deviation, ppm 2.0 Beta1 3.77

[0098] The CO₂ laser can be made with very high power and, since it hasa longer wavelength, a lower scale factor, which may be convenient,because it permits lower modulating frequency for a given maximumvelocity.

[0099] In one embodiment of the invention, the laser light is notvisible to the naked eye, but is readily so to a CCD or CMOS videocamera, including camcorders. Thus it is possible to verify that theactual target is the intended target, unlike radar based systems.

[0100] One embodiment of the invention is also used in the Type 2system. The Type 2 system requires the laser output to be split into twobeams that are recombined on the target and a detector be used tomonitor the reflection from the target. They can be built using theseprinciples by putting unequal delays in the two arms. A way of doingthis is to launch the laser output into an optical fiber. The power canbe split into two paths using a power splitter. The two paths then havedifferent lengths and illuminate the target. The differential pathlength is ^(t) _(D) in the above equations.

[0101] Another embodiment of the invention uses a separate detector 350,as shown in FIG. 3, instead of using the laser as both the source anddetector. The power supply, modulator and Doppler signal processor arenot shown in FIG. 3. The laser output from the laser diode 300 is splitinto two paths by a power splitter 310: one going via a circulator 320and a collimator 330, reflected by the target (not shown), back throughthe collimator 330 and circulator 320 to the power combiner 340 to thedetector 350. The other path leaves the power splitter 310 and iscombined with the first path at the power combiner 340 and, along withthe first path to the detector 350. The circulator 320 is used to routethe reflection from the target away from the laser diode 300 and towardthe detector 350. This implementation lends itself to the use of fiberoptic components. The same signal processing scheme as described abovecould be used to extract the Doppler velocity information. Oneembodiment of the invention may insert an isolator between the laserdiode 300 and the power splitter 310 to prevent energy from beingreflected back into the laser diode 300.

[0102] In one embodiment, a separate frequency-modulating section may belocated inside or outside of the laser diode 300. In one embodiment, oneof the two (frequency-modulated) laser signals and one reflected signalfrom the target 250, which have different signal path from each other,can be sampled and provided to the detector 350. In another embodiment,the detector 350 can receive and combine one of the emitted lasersignals and one reflected signal. In another embodiment, a separatecombining section, which may be located internal or external to thedetector 350 can perform a combining function such as the multiplying ofthe sampled signals.

[0103] Another embodiment of the invention uses separate transmit andreceive collimators 330 and 360, as shown in FIG. 4. This implementationuses completely independent paths for transmit and receive with onlylocal oscillator power flow connecting the two. Again, the power supply,modulator and Doppler signal processor are not shown in FIG. 4. Thelaser output from the laser diode 300 is split into two paths by thepower splitter 310. This time, the first path of the laser output is fedto the transmit collimator 330, reflected by the target (not shown),back through the receive collimator 360 and the power combiner 340 tothe detector 350. The other path leaves the power splitter 310 and iscombined with the first path at the power combiner 340 and, along withthe first path to the detector 350. The same signal processing scheme asdescribed above could be used to extract the Doppler velocityinformation.

[0104] Another embodiment of the invention uses a transmit/receiveswitch 370, as shown in FIG. 5. There may be times when it is desirableto turn the transmitter on and off. One situation when this may bedesirable is in a fog or rain. If transmission is continuous, thereflection from the fog or rain near the system may overwhelm thereceiver, preventing the system from responding to more distancetargets. With this version, square wave modulation is desirable insteadof sine wave. In this embodiment, the switching and modulation may bedone together so the laser frequency is one value during receiving andanother during transmitting. It is noted that the laser diode 300 maynot be tuned off as it will lose its coherence.

[0105] The above systems may use one or two collimators as desired,depending upon the option chosen in various situations.

[0106] If the delay of the echo is one half the transmit-receive cycletime, there will be no received echo. The echo returns during thetransmit time. This is called a range hole. The range holes may bereduced in significance by using a pseudorandom sequence to determinethe transmit receive state.

[0107] If its delay is small compared to the transmit-receive time; thereceived signal to the signal processor is nearly independent of range.This is because the instantaneous receive signal voltage, during theshort time it is present, is inversely proportional to range. But thewidth of the pulse is proportional to range. Combining these givessin(x)/x, where x=T,π/T,T_(r) is the receive duration and isproportional to the target range, and T is the transmit-receive cycletime. This holds until T_(r)=T/2, when some of the received echo startsto fall into the next range hole.

[0108] The invention can be applied to the sensing of the speed oramount of material extruded through a die, sheet metal through a roller,cable on (or off) a spool, speed of a car, train or ball, speed of thesurface of a liquid such as water, or molten metals, including aluminum.That is, many applications of velocity measurement systems actuallymeasure length, by integration of the velocity measurement. For example,the laser Doppler velocimeter system is used to measure the speed of acable (or fabric or lumber or rope) that is moving under it. Byintegration of the velocity measurement, the length of the cable (orfabric or lumber, rope) may be determined. The length may be the primaryinterest of the user, not the primary measurement, velocity. If thelaser Doppler velocimeter is attached to a trailer (rail car, tractor),it can measure the speed of the trailer. By integration, the distance ofthe path traveled may be determined. If the heading and originalposition is added, the present position may be determined. This is knownas dead reckoning.

[0109] It is also noted that speed or velocity may be integrated tobecome distance, so that even though the primary measurement is velocityor speed, distance or amount follows right behind. Plural applicationsare described below in more detail.

CONFIGURATION OF LASER BEAMS FOR DIFFERENT APPLICATIONS

[0110] Like all Doppler based systems, to measure three components (u,v, w) of velocity requires at least three beams pointing in differentdirections. The velocity components u, v and w are in the direction ofx, y and z respectively. To simplify things, V, X and F are used torepresent vectors containing estimates of the components of velocity,position or location change and Doppler frequency, and V_(i), X_(i) andF_(i) are used to represent the ith component of velocity, positionlocation change, and Doppler frequency.

[0111] The Doppler frequency is calculated from the velocity asF(t)=2λAV(t), where A is an NxM matrix made of the direction cosines ofthe Doppler beam directions and N is the number of beams and M is thenumber of components of velocity and is 1, 2 or 3. To get from themeasured frequency to velocity the following formula is appliedV(t)=A⁻¹F(t)/2λ, assuming that N=M where A⁻¹ is the inverse of A.

[0112] In the special case of a single beam with only one component ofvelocity, the velocity can be calculated from V(t)=F(t)λ/2Cos(θ), where0 is the angle between the beam direction and the direction of motion.

[0113] If the velocity is known, position or location change can becalculated by integrating velocity. The classic example is deadreckoning. Dead reckoning is how airplane pilots navigated before theadvent of modern navigation aids. The idea is that if the pilot knowswhere she was at the start of the flight and her speed and directionsince then, she knows where she is now. Mathematically, in order to getthe distance, including the direction from the velocity component, thecalculation that is required is integration.

[0114] The velocity components once measured can be integrated to obtainthe distance moved since velocity times time equals distance. Thus, ifthe beam's target or the Doppler system (or its mount) is moved, thedistance which is moved can be obtained by integrating the measuredvelocity with respect to time.

[0115] To show more clearly the integration the equations are:X(t) = ∫₀^(t)V(t)  t + X_(initial),

[0116] where X_(initial) is the initial location or amount, usuallyzero.

[0117] There are a number of applications for single beam systems. Thesecan be used to measure the amount or rate something is produced, forinstance. FIG. 6 shows a configuration for measuring the rate at which acable 640 is extruded through a die 620. A common method of doing thisis to have the cable go over a pulley and count pulley revolutions. Thisworks very well if precision is not required. As the cable goes over apulley it invariably slips by an unknown amount, creating anunderestimate of the amount of cable that went over the pulley. Also thecable jacket must have cooled enough that the jacket is hard enough notto be damaged as it comes in contact with the pulley.

[0118] It is assumed that, other than the angle θ, the beam emitted froma laser 660 is pointed in the direction of motion through a lens 680.The velocity is calculated from V(t)=F(t)λ/2Cos(θ). If this assumptionis incorrect, the error can be corrected by further dividing by Cos(φ),where φ is the amount of the beam which is misaligned with the cablemotion

[0119] The velocity measured is positive for a closing velocity, in thiscase as the material 640 is extruded. This equation and correction formisalignment are used for all the other single beam examples given here.In the unlikely event that the cable 640 moves back into the die 620, anegative velocity will be measured, indicating that the cable 640 didmove backward. When the velocity is integrated, the amount of the cable640 extruded will be obtained. Notice that it is desirable to place thesensor in a section of the cable 640 where the cable 640 is straight. Ifthe cable 640 is bent, for instance when it goes over a pulley (anotherpulley error), the cable 640 on the outside of the bend will get alittle longer and the length over measured and the cable 640 on theinside of the bend will be under measured.

[0120]FIG. 7 shows the use of a single beam to measure the amount orrate of material (sheet metal, for instance) as it leaves rollers 720 ina roller mill. FIG. 7 is a similar application involving measuring therate and/or amount of material leaving rollers. Again the velocity iscalculated from V(t)=F(t)λ/2Cos(θ). Notice that the laser 660 does nottouch the material. It is noted that only the laser energy is in contactwith the item that is being measured. This is desirable if the material700 is soft enough to be damaged if contacted by a roller or is hotenough to damage a sensor that is in contact with the material 700. Inrolling applications it is possible to roll different amounts ofmaterial at opposite ends of the roller 720. The laser Doppler methodpermits multiple sensors to be installed along the roller 720 to ensurethat the same amounts of material are made along the entire length ofthe roller 720.

[0121]FIG. 8 shows the use of a beam to measure the speed and/or amountof cable being transferred between two spools 760, 780. This is shownhere just to point out the option exists and the consequence that thesign will be negative for normal operation, because the cable 740 movesin the opposite direction relative to the beam of the previous examples.Of course the system could be designed to permit the sign to be reversedduring setup. Also the cable 740 should be straight at the measuringpoint to avoid errors.

[0122] In FIG. 8, the measurement is usually done by having the cable740 roll over a pulley and counting pulley rotations and is subject toslipping errors. This example shows the beam pointing in the oppositedirection compared to the cable motion as it flows from the supply spool780 to the take up spool 760. This is done for variety and to make thepoint that the sign of the motion and distance (length) will be oppositeto that of the other examples shown in FIGS. 6 and 7. It is possibleduring installation to set the direction of positive motion.

[0123]FIG. 9 shows a beam used to measure the flow of a liquid 800 in achannel. The liquid 800 could be a molten metal, water or any otherliquid. It is assumed that the laser Doppler system is aligned to thechannel and that the channel is straight. Using a laser Doppler systemhas an advantage in this case of not requiring the sensor to contact thefluid 800. The fluid 800 may be too hot or corrosive to allow a contactsensor to survive or function.

[0124]FIG. 10 shows a laser Doppler system mounted to a car (railroad,motor vehicle, military tank, etc.). In this case, the laser Dopplersystem is mounted to the vehicle. The usual way to build a speedometeror odometer is to estimate wheel rotation speed or to count wheelrevolutions. This solution has an added error factor because thevehicle's wheels 820 slip on the ground, road bed or rail 840.

[0125] It is noted that the sign of the measurement was flipped.Positive velocity is the distance between the Doppler System and itstarget getting smaller (closing velocity). In the other cases, theDoppler system was stationary and the target moving; in this case, it isthe other way, thus the sign changes.

[0126] To measure all three components of velocity requires three laserbeams. Two of an infinite number of possible configurations of laserDoppler beams for measuring these 3 components are shown in FIGS. 11 and13. It is not clear in the figures, but the beams are slanted downward(as θ in FIGS. 6 to 10) toward and focused on a surface or sheet whosevelocity is to be estimated. Alternately the surface could be stationaryand it is the velocity of the Doppler system or what it is mounted onthat is desired.

[0127]FIG. 11 shows an arrangement of three lasers 860-900 and theiroptics pointing in three different directions to get the threecomponents of velocity. This is a view from above. The beams are slanteddown toward the surface whose velocity is being measured. Thisconfiguration is handy if it is desirable to have the lasers and theiroptics near one another. It may be possible to share the same lens withthis configuration. This configuration requires that the target have thesame velocity at all three target locations, as is usually the case.

[0128]FIG. 12 shows an alternate arrangement of three lasers 910-930 andtheir optics pointing in three different directions to get threecomponents of velocity. This is a view from above. The beams are slanteddown toward the surface whose velocity is being measured. Thisconfiguration is desirable if the target is small or if the surfacevelocity is different at different locations, as usually is the case ifthe target is a fluid, because all the beams are focused at near thesame point.

[0129]FIG. 13 shows an arrangement of two lasers 940, 950 and theiroptics pointing in two different directions to get two components ofvelocity. The assumption is that the third component is known, usuallyzero. This is a view from above. The beams are slanted down toward thesurface whose velocity is being measured.

[0130]FIG. 14 shows four beams 960-990. This four (or more) beam systemis over determined and permits an error velocity to be estimated. Thefour beam system is known as the Janus configuration.

[0131] Systems using four or more beams allow an error velocity to beestimated because they are over determined. “Over determined” means thatthere are more beams than there are velocity components to measure.Using three beams to estimate two components is another example of the“over determined.” This permits the calculation of non-existent (in thereal world) velocity components that should be zero and, to the extentthey are not, indicate measurement error and can be used to judge theoverall velocity estimate.

[0132] In summary, reasonable applications of the invention include, butare not limited to, the following:

[0133] Measurement of the speed and direction of motion (two axis, using3 beams) of ground vehicles (tanks), which aid fire control ornavigation systems or permit dead reckoning,

[0134] Measurement of a closing velocity of two aircrafts or measuring aground velocity of an aircraft or a missile,

[0135] Speed gun,

[0136] Speed of rotating machinery,

[0137] Open channel liquid flow,

[0138] Non-contact vibration measurement from DC to many MHz, and

[0139] Integrating the LDV's output gives distance measurements. Thiscould be used to, measure the length of targets (cables, fabric, rope,)that move at the focal point.

[0140] One embodiment of the invention can sense ground velocities onthe order of micrometers per second (with small displacements, typicallya few nanometers) at acoustic frequencies from about 50 Hz to about 1kHz.

[0141] Another embodiment of the invention can use interferometric (orheterodyne) sensing of the reflected light for directing andmanipulating the laser beams internal to the sensor head.

[0142] Another application for the laser Doppler velocimeter is as alaser Doppler vibrometer. By directing the laser Doppler velocimeter ata vibrating target, the velocimeter can be used as a vibrometer. Whenthe velocimeter is pointed at the vibrating target, the measuredvelocity is the velocity of the vibrations of the target. If themeasurements of the velocity (sampling rate of the velocity) aresignificantly greater then twice the highest vibration frequency, a timeseries of the vibration velocity of the target is obtained.

[0143] Usually, the desired vibration information produced by avibrometer is the displacement of the vibration, not the velocity of thevibration. The displacement can be obtained by integrating the velocitytime series with respect to time. If a frequency spectrum of thevibration is desired, as is commonly the case, the Fourier transform ofthe velocity time series can be calculated. If the Fourier transform ofthe displacement is required, it can be obtained by dividing each valueof the velocity frequency spectrum by its frequency value.

[0144] This could be useful if a non-contact vibrometer is desired. Thiscould be also useful if non-contact is important; e.g., the target istoo soft, too hot, a liquid, or uneven to make contact with. Anothertype of application is that since it does not contact the target, it maybe used to scan or sweep across the surface target in search ofsomething.

[0145] Another application for the laser Doppler velocimeter is as amicrophone. By using the velocimeter as a vibrometer and pointing it ata membrane that is designed to be vibrated by sound waves, the output ofthe laser Doppler velocimeter is a time series of the sound pressurehitting the membrane, which is what a microphone does. If the walls of aroom or chamber are being vibrated by the sound waves on the other sideof the wall, this could be used to listen to the sound on the other sideof the wall.

[0146] While the above description has pointed out novel features of theinvention as applied to various embodiments, the skilled person willunderstand that various omissions, substitutions, and changes in theform and details of the device or process illustrated may be madewithout departing from the scope of the invention. Therefore, the scopeof the invention is defined by the appended claims rather than by theforegoing description. All variations coming within the meaning andrange of equivalency of the claims are embraced within their scope.

What is claimed is:
 1. A system for measuring velocity, comprising: apower source configured to provide a current; a laser emitting devicepowered by the current and configured to emit a laser signal; afrequency modulating section configured to frequency modulate the lasersignal; an optics assembly configured to split the frequency modulatedsignal into a plurality of laser signals, and to direct the splitsignals to a target and to receive at least one signal reflected fromthe target; a combining section configured to combine one of the splitsignals transmitted via one signal path and the at least one reflectedsignal transmitted via another signal path; a detector configured todetect the combined signal; and a signal processor configured to extractvelocity information from the detected signal.
 2. The system of claim 1,wherein the laser emitting device and the detector are configuredtogether in a laser assembly.
 3. The system of claim 2, wherein thefrequency modulating section is located in the laser assembly.
 4. Thesystem of claim 1, wherein the combining section is configured toreceive the one of the split signals and the at least one reflectedsignal, and to multiply the received signals so as to provide thecombined signal.
 5. The system of claim 1, wherein the laser emittingdevice comprises a laser diode.
 6. The system of claim 1, wherein thesignal processor is further configured to extract distance informationtoward the target from the velocity information.
 7. The system of claim1, further comprising a quadrature mixer configured to process thedetected signal adapted to be used in the signal processor, wherein thesignal processor is a conventional Doppler signal processor.
 8. Thesystem of claim 1, wherein the extracted velocity information comprisesthe magnitude and direction of the velocity of the target.
 9. A methodof measuring velocity, comprising: emitting a laser signal; frequencymodulating the laser signal; splitting the frequency modulated signalinto a plurality of laser signals; directing the split signals to atarget; receiving at least one signal reflected from the target;combining one of the split signals transmitted via one signal path andthe at least one reflected signal transmitted via another signal path;detecting the combined signal; and extracting velocity information fromthe detected signal.
 10. The method of claim 9, wherein the combiningcomprises sampling one of the split signals and at least one reflectedsignal, and multiplying the sampled signals.
 11. The method of claim 10,wherein the multiplying comprises summing the sampled signals andsquaring the sum.
 12. The method of claim 10, further comprisinggenerating a voltage signal that is proportional to the multipliedsignal.
 13. The method of claim 12, wherein the extracting comprisesprocessing the voltage signal so as to obtain a Doppler frequencyincluding sign.
 14. The method of claim 9, further comprisingcalculating distance information toward the target from the velocityinformation.
 15. A system for measuring velocity, comprising: a laseremitting device configured to emit a laser signal; a frequencymodulating section configured to perform frequency modulation on theemitted laser signal; an optics assembly configured to split thefrequency modulated signal into a plurality of laser signals, and todirect the split signals to a target and to receive at least one signalreflected from the target; a detector configured to detect a combinedsignal of one of the split signals transmitted via one signal path andthe at least one reflected signal transmitted via another signal path;and a signal processor configured to extract velocity information fromthe detected signal.
 16. The system of claim 15, wherein the laseremitting device and the detector are configured together in a laserassembly.
 17. The system of claim 15, wherein the detector comprises adetector diode.
 18. The system of claim 15, further comprising asinusoidal wave generator configured to generate a sinusoidal wave,wherein the frequency modulating section is configured to frequencymodulate the laser signal with the sinusoidal wave.
 19. A system formeasuring velocity, comprising: a laser emitting device configured togenerate a frequency-modulated laser signal; an optics assemblyconfigured to split the frequency modulated signal into a plurality oflaser signals, and to direct the split signals to a target and toreceive at least one signal reflected from the target; a combinerconfigured to receive one of the split signals transmitted via onesignal path and the at least one reflected signal transmitted viaanother signal path, and to multiply the received signals; and a signalprocessor configured to extract velocity information from the multipliedsignal.
 20. The system of claim 19, wherein the laser emitting deviceand the combiner are configured together in a laser assembly.
 21. Thesystem of claim 19, wherein the combiner comprises a detector.
 22. Amethod of measuring velocity, comprising: generating afrequency-modulated laser signal; splitting the frequency modulatedsignal into a plurality of laser signals; directing the split signals toa target; receiving at least one signal reflected from the target;combining one of the split signals transmitted via one signal path andthe at least one reflected signal transmitted via another signal path;and extracting velocity information from the combined signal.
 23. Themethod of claim 22, wherein the combining comprises receiving the one ofthe split signals and the at least one reflected signal, and multiplyingthe received signals.
 24. The method of claim 22, further comprisingdetecting the combined signal, wherein the extracting is from thedetected signal.
 25. The method of claim 22, further comprisingextracting distance information toward the target from the velocityinformation.
 26. A system for measuring velocity, comprising: a laserdevice configured to generate a frequency-modulated laser signal, thefrequency modulated signal being directed to a target and reflected fromthe target; a detector configured to detect a combined signal of thefrequency-modulated laser signal transmitted via one signal path and atleast one reflected signal transmitted via another signal path; and asignal processor configured to extract velocity information from thedetected signal.
 27. The system of claim 26, wherein the laser deviceand the detector are configured together in a laser assembly.
 28. Thesystem of claim 26, further comprising a combiner configured to receivethe frequency-modulated laser signal and the at least one reflectedsignal and to multiply the received signals so as to provide thecombined signal.
 29. The system of claim 28, wherein the detector isfurther configured to produce a voltage signal that is proportional tothe multiplied signal.
 30. The system of claim 26, wherein the detectorcomprises a detector diode.
 31. The system of claim 26, furthercomprising an optics assembly configured to split the frequencymodulated signal into a plurality of laser signals, and to direct thesplit signals to the target and receive at least one signal reflectedfrom the target.
 32. The system of claim 31, wherein the optics assemblycomprises: a power splitter configured to split the frequency-modulatedlaser signal into first and second split signals having different signalpaths from each other; and a collimator configured to collimate anddirect the first split signal to the target, and to collimate and directa reflected laser signal to the detector, wherein the detector isconfigured to detect a combined signal of the reflected laser signal andthe second split signal.
 33. The system of claim 32, further comprisinga circulator configured to receive and route the first split lasersignal toward the collimator and to receive and route the reflectedsignal toward the detector.
 34. The system of claim 32, wherein thecollimator comprises: a first collimator configured to collimate anddirect the first split signal to the target; and a second collimatorconfigured to collimate and direct a reflected laser signal to thedetector.
 35. The system of claim 34, further comprising a switchconfigured to switch the first split laser signal between the powersplitter and the first and second collimators.
 36. A method of measuringvelocity, comprising: generating a frequency-modulated laser signal,wherein the frequency modulated signal is directed to a target andreflected from the target; combining the frequency-modulated lasersignal transmitted via one signal path and at least one reflected signaltransmitted via another signal path; and extracting velocity informationfrom the combined signal.
 37. The method of claim 36, wherein thecombining comprises sampling the frequency-modulated laser signal and atleast one reflected signal, and multiplying the sampled signals so as toprovide the combined signal.
 38. The method of claim 37, wherein themultiplying comprises summing the sampled signals and squaring the sum.39. The method of claim 37, further comprising producing a voltagesignal that is proportional to the multiplied signal, wherein theextracting is from the produced voltage signal.
 40. The method of claim36, further comprising detecting the combined signal.
 41. A method ofmeasuring velocity, comprising: frequency modulating a laser signal, thefrequency modulated signal being split into a plurality of laser signalsand directed to a target, and reflected from the target; receiving oneof the split signals transmitted via one signal path and at least onereflected signal transmitted via another signal path; multiplying thereceived signals; and extracting velocity information from themultiplied signal.
 42. The method of claim 41, wherein the multiplyingcomprises summing the received signals and squaring the sum.
 43. Themethod of claim 41, further comprising producing a voltage signal thatis proportional to the multiplied signal, wherein the extracting is fromthe produced voltage signal.
 44. The method of claim 41, furthercomprising filtering an amplitude modulation component that has beenincidentally generated.
 45. A system for measuring velocity, comprising:a laser-emitting device configured to emit a laser signal; a frequencymodulating section configured to perform frequency modulation on thelaser signal, wherein the frequency-modulated laser signal is directedtoward a moving target; a power splitter configured to split thefrequency-modulated laser signal into first and second laser signals,wherein one of the laser signals is delayed with respect to the other; acombiner configured to receive and combine reflected signals from thetarget so as to provide a combined signal; and a signal processorconfigured to extract velocity information from the combined signal. 46.The system of claim 45, further comprising a detector configured todetect the combined signal.
 47. A method of measuring velocity,comprising: frequency modulating a laser signal, the frequency modulatedsignal being split into a plurality of laser signals; directing thesplit laser signals to a target; combining one of the split lasersignals with a reflected signal from the target; and extracting velocityinformation from the combined signal.
 48. The method of claim 47,wherein the combining comprises receiving the one of the split lasersignals and the reflected signal, and multiplying the received signalsso as to provide the combined signal.
 49. The method of claim 47,further comprising detecting the combined signal.
 50. A system formeasuring velocity, comprising: a laser assembly configured to generatea frequency-modulated laser signal, the frequency-modulated signal beingsplit into a plurality of laser signals and directed to a target, andreflected from the target, wherein the laser assembly is configured toreceive one of the split signals transmitted via one signal path and atleast one reflected signal transmitted via another path, and to combinethe received signals; and a signal processor configured to extractvelocity information from the combined signal.
 51. The system of claim50, wherein the laser assembly comprises: a laser emitting deviceconfigured to emit a laser signal; a frequency modulating sectionconfigured to perform frequency modulation on the emitted laser signal;and a combiner configured to receive and combine the one of the splitsignals and the at least one reflected signal.
 52. The system of claim50, wherein the laser assembly comprises: a laser emitting deviceconfigured to emit a laser signal; a frequency modulating sectionconfigured to perform frequency modulation on the emitted laser signal;and a detector configured to receive the one of the split signals andthe at least one reflected signal, and to multiply the received signalsso as to provide the combined signal.
 53. A system for measuringvelocity, comprising: means for generating a frequency-modulated lasersignal, the frequency modulated signal being split into a plurality oflaser signals and directed to a target, and reflected from the target;means for receiving one of the split signals transmitted via one signalpath and at least one reflected signal transmitted via another signalpath; means for multiplying the received signals; and means forextracting velocity information from the multiplied signal.
 54. A methodof measuring velocity, comprising: generating a frequency-modulatedlaser signal, the frequency-modulated signal being directed to a target;and measuring the velocity of the target based on a combination of thefrequency- modulated laser signal and a reflected signal from thetarget.
 55. The method of claim 54, wherein the measuring the velocitycomprises measuring the velocity of one of the following targets: amoving vehicle, a cable being extruded from a die, a sheet metal beingrolled through a roller, a cable being transferred between two spools, asurface of liquid, a vibrating object, a rotating machinery, or a groundmoving with respect to a vehicle.
 56. The method of claim 55, furthercomprising obtaining the distance toward the target from the measuredvelocity.
 57. The method of claim 54, wherein the measuring comprisessensing a ground velocity of the target on the order of micrometers persecond at acoustic frequencies from 50 Hz to 1 kHz.
 58. The method ofclaim 54, further comprising using interferometic or heterodyne sensingof the reflected laser signal.
 59. The method of claim 54, wherein themeasuring the velocity comprises measuring a closing velocity of twoaircrafts or measuring a ground velocity of an aircraft or a missile.60. A method of measuring velocity and distance, comprising: generatinga frequency-modulated laser signal, the frequency-modulated signal beingdirected to a target; measuring the velocity of the target based on acombination of the frequency- modulated laser signal and a reflectedsignal from the target; and obtaining the distance to the target by themeasured velocity.
 61. The method of claim 60, wherein the measuringcomprises measuring the speed and direction of motion of a groundvehicle that aids a fire control or navigation system, or permits deadreckoning.